1. Field of the Invention
The present invention relates to polyisocyanates containing allophanate groups and optionally isocyanurate groups, to a process for preparing these polyisocyanates and to their use as binders or binder components in coating compositions.
2. Description of the Prior Art
Polyisocyanates containing allophanate groups and their use as the isocyanate component in coating compositions is known and described in U.S. Pat. Nos. 4,160,080 and 4,177,342 and 4,738,991. Polyisocyanates containing allophanate groups and isocyanurate groups are known and disclosed in U.S. Pat. Nos. 5,124,427, 5,208,334, 5,235,018, 5,290,902 and 5,444,146. The preparation of these polyisocyanates is generally carried out by reacting excess quantities of preferably aliphatic or cycloaliphatic diisocyanates with monohydric or polyhydric alcohols in the presence of a catalyst that promotes the formation of allophanate groups or a catalyst that promotes the formation of both allophanate and isocyanurate groups. After the reactions are complete, the excess unreacted monomeric diisocyanates are removed by distillation and products are formed having properties which can be varied widely depending upon the diisocyanate used.
The preparation of polyisocyanates containing allophanate groups from aromatic diisocyanates is described in U.S. Pat. No. 3,769,318. One of the disadvantages of these aromatic allophanate polyisocyanates is their poor heat stability. During thin-film distillation the allophanate polyisocyanates regenerate the incorporated diisocyanates making it impossible to completely separate the excess diisocyanate following the allophanatization reaction.
With regard to allophanate polyisocyanates based on 4,4'-bis-(isocyanatocyclohexyl)-methane (HMDI), it is possible to separate the excess diisocyanate down to acceptable levels. However, just because the decomposition of the allophanate polyisocyanates may occur at a slower rate, such that it is possible to remove monomer, does not mean that the resulting polyisocyanates are heat stable. Any decomposition of the allophanate polyisocyanates is disadvantageous because it affects the yield, increases the amount of monomer that must be removed and also results in the presence of urethane group-containing compounds in the final product. When the allophanate polyisocyanates are prepared from monofunctional alcohols, the urethane group-containing compounds are monoisocyanates, which is even more disadvantageous because they act as chain terminators when the polyisocyanates are subsequently used for the production of coatings.
Accordingly, it is an object of the present invention to provide thermally stable polyisocyanates, which contain allophanate groups and are based on HMDI.
This object may be achieved in accordance with the present invention, which is described in more detail below. In accordance with the present invention the polyisocyanates containing allophanate groups are prepared by using different isocyanate components for the urethanization and the subsequent allophanatization and optional trimerization reactions.